Prostate motion management is critical for modern high-conformality dose-escalated prostate intensity-modulated radiotherapy (IMRT). Numerous clinical studies show that, in comparison to skin tattoo or bony anatomy alignment, use of gold markers in conjunction with pre-treatment portal MV or on-board kV imaging can significantly reduce the setup margins and therefore potentially lead to decreased normal tissue toxicity. Intrafraction prostate motion is considered to be a limiting factor on margin reduction, at least for patients with relatively large intrafraction motion. Because intrafraction prostate motion is generally more unpredictable than respiration motion, techniques developed for respiratory motion correction, such as gating based on external surrogates, are often not directly applicable.
Although recently developed electromagnetic transponders provide real-time 3D localization without radiation dose, they are physically much larger than the gold markers conventionally used in radiographic tracking and produce severe MRI artifacts, hindering MR-based post-treatment assessment. Real-time simultaneous acquisition of two X-ray images from different viewing angles for 3D target localization has also been implemented with sub-millimeter accuracy, e.g., fluoroscopic imaging using two kV systems, and MV-kV imaging using cine-MV and fluoroscopic kV systems. Stereoscopic X-ray imaging with one or two kV sources, however, poses the problem of accumulating excessive patient imaging dose. A reliable reduction of kV beam use would be highly desirable.
The prostate is mostly stationary and drifts slowly with abrupt motion occurring only occasionally. What is needed is a minimally invasive method to detect only motion that is potentially beyond a pre-defined threshold. Then assess the over-threshold event and obtain more accurate position information to use for interventional motion correction.